1. Field of the Invention
The present invention relates to a navigation system that displays map information and a method of displaying map information in a quasi-three-dimensional manner.
2. Related Art
In general, a vehicle navigation system detects a present position of the vehicle, reads out map data corresponding to an area surrounding the present vehicle position from an information storage medium such as a CD, DVD, etc., and displays the map information surrounding the vehicle position on a display screen. Further, the system displays a vehicle position mark that indicates the present position of the vehicle at a specific position on the screen, and scrolls the map information of the area surrounding the vehicle putting the vehicle position mark in the center, in accordance with the travel of the vehicle, whereby a driver is able to comprehend the map information surrounding the vehicle position at any time.
Further, a recent navigation system sets an observing point with a specific position high above the vehicle position, and uses a known technique that displays the map information along the vehicle travel direction from a bird""s-eye view. Thus, by providing a bird""s-eye view display, the system is able to present seemingly three-dimensional map information that gives a user an extensive view. Accordingly, the system allows the user to receive more information than presenting the map information on a plane (two-dimensionally), and it becomes easier for the user to confirm the travel direction of the vehicle, which is advantageous.
In the bird""s-eye view display, the map information near the lower part of the display screen is expanded to a maximum degree, and the most important map information surrounding the vehicle position, which is placed at or slightly under the screen center, reduced accordingly and becomes difficult to see, which is a problem.
Further, a precise execution of the bird""s-eye view display will require calculations of the trigonometric function using the angle along the visual field direction. Because the calculations need to be executed for each display pixel, a display corresponding to the vehicle position that varies every moment will impose a very heavy burden on the calculations, which is also a problem. Also, the display positions of various mark images and character information, etc., which are superimposed on the map data, need to be calculated before executing the bird""s-eye view display, which also requires the calculations of the trigonometric function, imposing a still heavier burden.
The present invention has been made in view of these circumstances, and it is an object of the invention to provide a navigation system that lightens the burden of the calculation processing and also achieves a display of the detailed information of the most important area surrounding the vehicle position.
In order to accomplish the foregoing object, the navigation system of the present invention stores in a map data storage means map data having a uniform reduction scale factor, necessary for drawing map information. On the basis of the map data read from the map data storage means, a map drawing means draws the map information in a drawing memory with regard to a first area including a screen center, and, with regard to a second area surrounding the first area, draws the map information in the drawing memory with a smaller reduction scale factor than that in the first area.
Generally, the navigation system lays out the vehicle position on the center area of the display screen or slightly lower. Therefore, by drawing the map information of the second area surrounding the first area with a smaller reduction scale factor than that for the first area including the screen center, the navigation system of the invention makes it possible to display detailed map information in the most important area surrounding the vehicle position. In the second area remote from the vehicle position, the system makes it possible to display map information including more remote information.
Since the system draws the map information corresponding to the second area by very simple calculations that reduce the map information on the basis of the specific reduction scale factors, the system does not require complicated calculation processing using the trigonometric function as in case of the bird""s-eye view display, and thereby reduces the burden of the processing to a great extent. Further, the system uses map data having a uniform reduction scale factor, which is the same as the map data that a general navigation system uses. Therefore, it does not need to create new map data for drawing the map information, and is able to use the existing map data as it is, which is advantageous.
It is preferable that the first area is rectangular, including the screen center and the lower area thereof. The map drawing means draws the map information in the drawing memory with a reduction scale factor only in the horizontal direction as to areas on both sides of the first area, with a reduction scale factor only in the vertical direction as to an area directly above the first area, and with reduction scale factors in both the vertical and horizontal directions as to the slant upper areas relative to the first area. Since the system reduces the map information in this manner, it is able to display quasi-three-dimensional map information having a feeling of depth, resembling the bird""s-eye view display.
According to the method of displaying quasi-three-dimensional map information of the present invention, map data corresponding to an area wider than an area required for displaying the map information with an equal scale factor is read. Next, the map information is drawn, setting a horizontal reduction scale factor for areas on both sides of a center area to a first value smaller than a first reference value, while bringing the horizontal reduction scale factor for the center area into correspondence with the first reference value and bringing a vertical reduction scale factor for an area lower than the center area into correspondence with a second reference value, and setting the vertical reduction scale factor for an area above the lower area to a second value smaller than the second reference value. Finally, the drawn map information drawn is displayed on the display screen.
Thus, with regard to the specific area lower than the center area of the display screen, since the map information can be drawn with the equal scale factor, setting the vehicle position in this specific area achieves a display of detailed map information in the area surrounding the vehicle position. In setting the area where the map information is drawn with the equal scale factor in the center, as to the areas on both sides of this area, the map data are reduced in the horizontal direction. As to the area above this center area, the map data are reduced in the vertical direction. As to the areas located obliquely above this center area, the map data are reduced in both the horizontal and vertical directions. Therefore, map information that allows a user to view remote places, that is, the quasi-three-dimensional map information, can be expressed through simple calculation processing.